A recipe from two eras: How conifers ward off their enemies

(Press-News.org) To the point:

Conifers use resin to protect themselves against pests. This resin contains diterpenes, which are defensive substances.  Some of these diterpenes originated over 300 million years ago, before conifers evolved. Other diterpenes developed independently in different conifer species much later, presumably to protect against bark beetles. This repeated evolution was only possible because enzymes that produce diterpenes had previously undergone changes that unlocked evolutionary pathways towards certain substances. This is based on a mechanism called “epistasis”, which allows new traits to evolve once preparatory changes have already occurred in some cases. The findings provide insight into the evolutionary mechanisms of conifer defenses and could help us better understand and utilize natural plant protection.  

Conifers, such as pines, spruces, and firs, produce sticky resins that protect the trees from insects and pathogens. Important components of this resin are diterpenes, special natural substances that repel bark beetles and fungi. The enzymes that produce these compounds are called diterpene synthases.

A research team at the Max Planck Institute for Chemical Ecology in Jena, Germany, and Iowa State University in Ames, Iowa, USA, wanted to find out whether these enzymes originated once in the distant past or evolved independently in different conifers more recently. "Diterpene synthases are exciting enzymes because even minor structural changes cause them to produce different chemical products. They are therefore ideal for investigating how plants came to produce such an enormous variety of defense substances over the course of evolution," said Andrew O'Donnell, the study's first author from the Department of Biochemistry, explaining the research's starting point.

 

A journey into the evolutionary past of enzymes

To unravel the evolutionary history of these enzymes, the team performed genetic analyses to reconstruct probable ancestral diterpene synthases and study them in the laboratory. The scientists modified these reconstructed enzymes to observe how their products changed.  "To determine an enzyme's products, we transferred its genes into bacteria. The bacteria then produced the enzyme for us. We isolated the enzyme, added suitable starting materials, and analyzed the resulting products in detail using modern analytical methods," explained Axel Schmidt, head of the Conifer Defense Project Group.

In order to determine the age of certain ancestral enzymes, the researchers had to take into account the sequences of numerous diterpene synthases as well as the evolutionary relationships among conifer species. The result: Some of the diterpenes found in today's conifer resin originated 300 million years ago, long before pine, spruce, and fir trees existed in their current forms. Other important diterpenes, however, developed more recently and independently in several different tree species.

 

Why evolution sometimes takes a very long time

This raised the question of why some of these compounds took so long to develop – yet still led to similar results in different tree species. A genetic mechanism called epistasis played a central role in this process. New traits often emerge only if other changes occurred beforehand. "The potential for plants to develop certain substances increased slowly over millions of years and then dramatically after conifers separated from other plants. This could explain why some plant groups develop the same characteristics repeatedly," says Andrew O'Donnell.

 

Protection against bark beetles

Today, conifer resin is a mixture of ancient and more recent diterpenes. These more recent defensive substances may have developed when bark beetles already existed, as supported by fossil findings. Despite following different evolutionary paths, pine, spruce, and fir trees probably developed identical diterpenes independently as a defense against these pests.

This unique blend of ancient and more recent defensive substances may be crucial to how well trees fend off current pests, such as bark beetles. "A tree's ability to adapt quickly to new challenges, such as bark beetle attacks, depends on the changes that have already occurred in its metabolism over the course of evolution. This prehistory determines which new characteristics can develop — and thus, how well the plant adapts," explains Jonathan Gershenzon, head of the Department of Biochemistry.

The researchers now want to investigate how the evolution of diterpenes and diterpene synthases has influenced the trees' ability to defend themselves today against both bark beetles and their associated fungal species. A mixture of substances is likely needed to most effectively defend against the dual threat of beetles and fungi.

 

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